High dose rate (HDR) brachytherapy is a very promising method for treating different kinds of tumors. One or more catheters are inserted directly into one or several tumor(s) in a minimally invasive procedure. Through these catheters, small radioactive seeds can be shifted to pre-defined positions in order to deposit a high radiation dose precisely in the lesion with only minimal damage of surrounding healthy tissue compared to other focal techniques. At the same time HDR brachytherapy allows treatment of larger tumors and is unlike Radio-Frequency Ablation (RFA) not prone to cooling effects of nearby vessels. HDR brachytherapy allows accurate focal treatment in deformable or even moving tissue.
Current treatment planning software optimizes the dose distribution after the intervention, when the catheters are already placed in the patient and the positions are fixed. The parameters which can then be varied are the dwell positions and the dwell times of the radioactive seed being shifted through the catheters.
Besides time-consuming graphical interactive optimization algorithms, where these parameters are set manually, a forward calculation of the dose distribution is performed to check if all constraints are fulfilled, very efficient inverse planning algorithms have been developed which are able to find the optimal distribution of dwell times and positions in less than a minute.
While catheter placement is conventionally performed under ultrasound guidance or using guiding templates fixed to the patient, new real-time MR-guided techniques offer a much higher functional specificity and spatial precision. With the development of MR-guided interventional procedures, ways of further optimization of the treatment open up.